1. Field of the Invention
The invention concerns a catadioptric projection objective with an object plane, a physical beam splitter, a concave mirror, a image plane, a first objective part, a second objective part, as well as a third objective part.
2. Description of the Prior Art
Optical projection systems with great resolution are used for projection exposure devices, with which patterns of photomasks or reticles, which are designated below generally as masks or reticles, are projected or exposed on semiconductor wafers or glass plates coated with photosensitive materials. In order to further reduce the structural width, which is essentially limited by the diffraction-limited resolution, preferably DUV and VUV radiation with a wavelength xe2x89xa6260 nm are used for projection of such small structures. Catadioptric systems are used for such projection devices as projection objectives, among others. Catadioptric systems permit a large bandwidth, whereby the costs of the laser exposure source can be kept small and the efficiency is high.
A catadioptric projection system with intermediate image has been made known from U.S. Pat. No. 5,636,066. Beam splitting is produced geometrically by means of a deflecting mirror, which is perforated in the center of the mirror, and represents a diaphragm for the beam reflected by the catadioptric part at the concave mirror.
The system according to U.S. Pat. No. 5,636,066 is constructed in such a way that the intermediate image is formed in the diaphragm plane of the deflecting mirror.
Another system with geometric beam splitter, which is designed as a deflecting mirror, has become known from U.S. Pat. No. 5,691,802. In the projection objective known from U.S. Pat. No. 5,691,802 with an intermediate image, the intermediate image is formed in front of the catadioptric objective in the first objective part. A disadvantage of the system known from U.S. Pat. No. 5,691,802 is the fact that this necessarily involves an eccentric system due to the geometric beam splitting.
A system with a physical beam splitter has become known from EP-A-0 475,020. The system shown in EP-A 0 475,020 comprises at least one catadioptric input system as well as a dioptric output system. The mask to be imaged is applied directly onto a beam splitter, which is preferably a prism. A part of the light reflected by the catadioptric system is deflected to the dioptric system by means of the beam splitter.
A disadvantage of the arrangement according to EP-A 0 475,020 is the fact that the object to be imaged is arranged directly on the beam splitter and the correction of the intermediate image limits the possibilities for correction of the entire system.
U.S. Pat. No. 4,302,079 is shows a system with a polarization-optical beam splitter. The change of the direction of polarization of the beam reflected by the concave mirror in the catadioptric objective part is varied by means of induced double refraction.
U.S. Pat. No. 4,896,952 shows a system with polarization-optical beam splitter, whereby the change of the direction of polarization in the catadioptric objective part is achieved by means of a xcex/4 plate.
DD-C-215,179 also shows a system with a physical beam splitter, which is formed as a partially transparent beam-splitting prism. The system according to DD-C-215,179 has two identical mirrors on two of the four sides of the beam-splitting prism perpendicular to a pregiven plane as well as two dioptric structural groups on the other two sides of the beam splitting prism. The dioptric structural groups are designed in such that the Petzval sum of the two systems largely compensates for that of the mirror.
A catadioptric projection objective without intermediate image has become known from EP-A 0 350,955, in which a first lens group or a first objective part is provided between the object, e.g., the reticle, and the physical beam splitter, a second lens group is provided between the physical beam splitter and the concave mirror, and a third lens group is provided between the physical beam splitter and the image plane.
U.S. Pat. No. 5,808,805 and U.S. Pat. No. 5,999,333 show a catadioptric objective with intermediate image and beam splitter as well as at least two partial objectives, whereby the partial objectives are constructed such that the intermediate image lies in the vicinity of the beam splitter surface of the physical beam splitter. According to U.S. Pat. No. 5,808,805, a physical beam splitter, for example, a beam-splitting prism, is used as the beam splitter; U.S. Pat. No. 5,999,333 also shows the use of a mirror as a geometric beam splitter.
A disadvantage of the system with a geometric beam splitter is the fact that this involves an eccentric system. Both the system known from U.S. Pat. No. 5,808,805 as well as that from U.S. Pat. No. 5,999,333 have a very high range of angle of incidence of the returning beams reflected by the concave mirror and impinging on the surface of the beam splitter.
U.S. Pat. No. 5,861,997 shows a system similar to U.S. Pat. No. 5,808,805 as well as of U.S. Pat. No. 5,999,333 with two intermediate imaging systems, whereby one intermediate image lies in the vicinity of the beam splitter, so that high angles of incidence occur at the beam splitter.
The main disadvantage of the objectives known from U.S. Pat. No. 4,302,079; U.S. Pat. No. 4,896,952; EP-A-0 350,955; DD-C-215,179; U.S. Pat. No. 5,808,805; U.S. Pat. No. 5,861,997; and U.S. Pat. No. 5,999,333 is the fact that the beams impinging on the beam splitter layer have a broad range of angles. This is true particularly for the embodiments shown in U.S. Pat. No. 5,808,850; U.S. Pat. No. 5,861,997; and U.S. Pat. No. 5,999,333, in which the intermediate image is formed in the vicinity of the beam-splitter surface.
The image quality is reduced by the large range of angles of the radiation impinging on the beam-splitter layer, since the reflectivity and the transmission of the beam-splitter layer depend on the angle of incidence onto the beam-splitter layer and insofar different intensity distributions result for different incidence angles.
In order to avoid this disadvantage, EP-A-0 602,923 proposes providing a lens in front of the physical beam splitter, by means of which the radiation impinging on the beam splitter is made parallel.
A parallel beam path is also produced in the catadioptric projection system known from U.S. Pat. No. 5,771,125. A disadvantage of the objectives according to EP-A 0 602,923 and U.S. Pat. No. 5,771,125 is that the positive refractive power of the mirror is not compensated for in the catadioptric part. This means that in fact, the beam impinging on the beam-splitter layer is made parallel, but not the beam returning after reflection at the concave mirror. The beam-splitter layer is then loaded in a direction under larger aperture angles. This in turn has the consequence that a pure splitting into the polarization directions cannot be achieved. Double images and a loss of contrast result from this.
In order to minimize the angle of incidence onto the beam-splitter layer, DE-A 4,417,489 proposes arranging at least one convergent lens for making the light beam impinging on the beam-splitter layer parallel in the case of a catadioptric projection system with physical beam splitter on the object side in front of the physical beam splitter, and a divergent lens group with one divergent lens after the physical beam splitter in the catadioptric objective part, in order to compensate for the effect of the convergent lens for making the beam impinging on the beam-splitter layer parallel. Further, another convergent lens is provided on the image side after the beam-splitter prism, in order to compensate for the effect of the divergent lens group in the case of a beam returning from the concave mirror in double passage.
The disadvantage of the arrangement according to DE-A 4,417,489 is the fact that the correction of longitudinal chromatic aberration (CHL) is insufficient due to the over-correcting catadioptric objective part.
A first object of the invention is thus to provide a catadioptric projection objective, which overcomes the disadvantages of DE-A 4,417,489, and particularly allows for a complete correction of the longitudinal chromatic aberration (CHL). According to the invention, this object is solved in a first embodiment by a projection system, in which more negative refraction power is arranged in the second objective part between beam splitter and concave mirror. A splitting of this high negative refraction power into at least two negative lenses is advantageous.
In an alternative embodiment, the second objective mirror is over-corrected relative to the chromatic length aberration CHL, while the first and third objective parts provide an under-correction, so that the over-correction of the second objective part in double passage compensates for the under-correction of the first and third objective parts up to at least 70% and preferably to more than 85%.
The advantages of such a projection objective are:
a. the correction of the of longitudinal chromatic aberration (CHL) is no longer limited;
b. the working distance between both the object plane and the first objective part as well as between the third objective part and the image plane is sufficient for an application in microlithography;
c. a double parallel beam path is produced on the beam-splitter surface both in passage to the concave mirror as well as in the return from the concave mirror;
d. the entire system can be constructed on-axis;
e. in the design with intermediate image, a conjugated, accessible diaphragm plane can be provided in the third objective part.
In a first embodiment of the optical system, the divergent lenses of the second objective part are spatially separated from one another.
A polarization-optical beam splitter with polarization-dependent reflecting layer system is preferably used as the physical beam splitter. The use of obliquely positioned planar plates would also be possible.
It is particularly preferred if the physical beam splitter is formed approximately in cube shape, i.e., the side ratio A1:B1 of the surface of the physical beam splitter pointing to the object is in the range of 0.7 less than A1:B1xe2x89xa61.0 and the side ratio A2:B2 of the surface pointing to the wafer also lies in the range of 0.7 less than A2:B2xe2x89xa61.0.
A particularly good imaging quality is achieved, if the second objective part and the concave mirror as well as the physical beam splitter are arranged in vicinity to the system aperture.
If the system is configured as a system with intermediate image, then advantageously the entire objective part has an imaging scale xcex2intermediate of 1xc2x10.7, preferably 1.5, from the object up to the intermediate image, thus the first as well as the second objective parts and concave mirror in double passage and the third objective part up to the intermediate image.
In order to focus the principal beam to the vertex of the concave mirror, it is advantageously provided that the first objective part comprises a field lens.
A reduction of the structural length of the objective results if one or more deflecting mirrors are provided. A deflecting mirror arranged after the physical beam splitter in the third objective part is preferred. Advantageously, the reticle and wafer are made parallel by introducing a deflecting mirror. A reduction in the number of lenses in the third objective part can be achieved, if aspherical surfaces are provided in the third objective part. This is true also for the second objective part and the first objective part comprising the field lens. An arrangement of the deflecting mirror in the first objective part would also be possible.
In a particularly advantageous embodiment the catadioptric projection objective is formed with intermediate image and has a telescopic configuration with a convergent lens in the first objective part in front of the physical beam splitter, a divergent lens, and a convergent objective after the physical beam splitter in the third objective part and in front of the intermediate image, in order to largely making parallel the radiation impinging on the beam-splitter layer.